HMV 904
Replica

The following is by Victor Barker, who has built this very nice
replica of a prewar HMV 904:

It was 1958
when I was a teenager and living with my parents near London
that I was given an HMV 904 T.V. to experiment with. I
managed to get it working by repairing the vertical timebase and replacing a couple of
valves, the mixer and the sync limiter. Unfortunately my
father was conned into selling it to a associate of his, much to my displeasure. In those days it was the parents who
were in charge. I was at the time apprenticed to an
electronic instrument manufacturer making R.F. test gear for
the BBC amongst others for T.V. transmitting stations and
receiver manufacturers. I was very interested in the
technique of television engineering and the bug is still
with me.

Having moved to Australia during 1968 I had little chance of
finding any old British 405 line T.V. receivers to form a
collection but as the years went by I was able to find a few early post war sets when visiting my family in
the U/K but never an HMV 904.

About 7 years ago I received an email from a Doctor in Queensland,
Hugo Holden who had seen some articles I had posted on the
internet describing various aspects of early television broadcasting and engineering, he
told me about an HMV 904 that he had totaly restored, that
was followed by a visit to his home in Queensland. I was immediately fired up with the desire to
either obtain such a set or build one. I am not sure what
would have been the best choice but I decided to start
gathering parts to build one. The build philosophy was to
copy the original circuit as far as possible using
components that would not add or detract from the original
design performance, this entailed using valves (tubes) of
similar performance and preferably made or in common use
before the WW2. Rectifiers and audio valves would not
present a problem, however R.F. I.F. and video types were a
problem.

It would have been possible to locate a set of original valves with
difficulty, perhaps even a set of spares but with little
guarantee of their condition. I therefore decided to use valves designed pre war and made during the
war by the thousand for military use. I chose the British
SP61/VR65 a direct copy of the SP41 the only difference being that the SP61 employed a 6.3V heater rather
than a 4 V heater. The valve had a higher gain than the
original Marconi MSP4 employed as R.F. amplifier but a lower gain than the three KTZ41s used as I.F.
amplifiers, the MSB4 active video detector also has a lower
gain than the SP61. The total conductances in practice with
this set using SP61s total about the same as the original
valve complement and the performance of the finished set is
similar to the original 904.

All the octal valves were as in the original or their direct
equivalents whilst the pulse diode a D42 was changed for an
EB34 double diode, the two sections being wired in parallel. The X41 with its B7 pin base has been
retained. The KT41 sound output valve has been changed to an
octal 6V6. The rectifiers are as original. In all cases where required slight changes to cathode bias
resistors have been permitted, these have been minor and few
in number to accommodate the different slopes of the SP61s
in three cases and the vertical output valve required a
different bias resistor for another reason as will be
explained later in the article.

The
original CRT was a major stumbling block because of the near
if not total impossibility of obtaining one, various 5 inch
tubes were tried but only two types were in any way acceptable. The tubes were both of American
origin being the 5AXP4 a non aluminised electrostatic test
tube and a 5FP4 a magnetic focused white screen tube very similar to the very common 5FP7 but designed for
TV camera viewfinder use. The latter tube was the type
finally used, it performs flawlessly and even at the very low E.H.T. of 2.5 KV employed in the 904 design is not
the limiting factor determining the picture resolution. The
brightness is adequate under subdued lighting as with the original set using an EMISCOPE 3/1.

At
the time of construction access to any coil winding
information was not available and still isn't I used valve
data, circuit fixed capacitances and guestimates to determine the total circuit capacitances in order to design
the R.F. and I.F. coils. Some degree of empiric
determination was used in a few areas. Aluminium tuning
cores were used where variable permeability was required.

The
tuning scale mechanism employing an epicyclic drive was
engineered to the same spec as the original set, the logging
dial is somewhat simpler at present but designed to be
modifiable if I can find suitable parts to replicate the
original chain drive. The dial was made from a photograph of
an original dial that was re- drawn using cad program on a
P.C. and finally printed onto photographic paper and used as
a rear dial plate rather than printing onto the rear of the
glass frontplate.

All laminated transformers were hand wound or modified types except
for the audio output transformer, it was a standard 4500/4
Ohm radio output transformer to suit a 6V6. The focus coil is entirely home wound whilst the
present deflection coil assy is a commercially made unit.

All nut and bolts used on the T.V chassis are zinc plated steel
cheese head 2, 4 and 6B.A. types, whilst the screws used in
the cabinet construction are counter sunk brass types.

Most
of the wiring unfortunately is carried out using plastic
covered wire, whilst many of the short interconnects are
wired with 22 s.w.g. t.c.w. and sleeved with mil grade black
cotton impregnated sleeving removed from wartime radar
equipment of British design. The tagboards are of wartime
vintage and also British, they are almost identical to the
ones used in the original 904, they appear to be made by the
same company. How lucky can one be?

The
tuning capacitor was removed from an old Australian radio
but was made by the same company that made the 904 unit even
the direction of rotation and the fixing holes I have
subsequently discovered are the same. Different trimmers
were fitted to accommodate local requirements. The multiple
electro housed in a rectangular metal box was faithfully
reproduced using modern electros of higher voltage rating.

I
have employed different sockets for the inputs at the rear
of the chassis to match the banana plugs that were available
at the time of construction, I refer to the aerial panel.
Cotton covered mains cord is employed to enhance the
appearance but does employ an earth lead to conform to local
safety regulations, the original set did not, a separate
earth was recommended.

Circuit differences are minor and few in number, they are
essentially: the use of a vertical output transformer
because I have not been able to find suitable wire to make
the very high impedance vertical coils needed in the
deflection yoke when resistive/capacitive coupling to the
output valve is employed as in the original design. When a
suitable yoke becomes available the transformer will be
removed and the circuit retuned to the original arrangement.
The circuit is otherwise unaltered except for a change in
the output valves bias resistor. Use of silicon diodes is
employed in a voltage doubler at present for the E.H.T.
supply because the only available
transformer to hand had been wound with a 900V secondary.
however the original E.H.T. rectifier has been installed on
the chassis so that in the event of making or obtaining the correct deflection coils I will revert to the
original arrangement. The H.T.supply is wired to operate
with either a valve rectifier or silicon rectifiers incorporating series resistors. The latter is to avoid the
danger of having a very hot rectifier at the rear of the set
whilst the set is on display and operating. Non of the above changes in any way affect the performance of the set.

The only real departure from the original set is the use of high
quality capacitors in place of the original paper type,
their effect being only their vastly longer longevity.

The design of the cabinet presented a number of problems being my
first attempt at cabinet making, a direct copy was ruled
out, instead it was my opinion that the appearance should
show clearly that no attempt was made to make a copy but
rather to make a similar cabinet with no doubt as to the
origins of the design. The top of the cabinet showing the
most prominent difference. The dimension are the same as the
original and the timber is solid maple, no veneer is used.

The
chassis layout is a direct copy of the original and the
dimensions are nearly the same, the exception being that the
chassis is almost half an inch higher to accommodate one of the power transformers.

Obtaining suitable control knobs was solved by having the
volume and tuning knobs made from aluminium and machined on
a lathe whilst the other knobs were standard radio knobs
suitably drilled and modified where required for the
concentric arrangement for the contrast and brightness and
the vertical and horizontal hold controls. All knobs were
dipped in enamel paint to give them the original bakelite
appearance. The escutcheon around the radio dial is made
from thin timber as found in
cigar boxes and hand painted with many coats of gloss enamel
paint.

The underside of the cabinet has the same system of plinths as the
original also the CRT mask is a copy of the original and
made from timber.

A
few comments about the original design may well be in order.
Firstly I have every admiration for the designers of the
time. The development of the circuitry during 1937 shows how
well they mastered the myriad of problems they were faced
with. The sync separator for one is quite a clever
arrangement whereby a diode suitably biased increases the
effective video detector anode load during the sync pulses
thereby more than doubling the sync pulse amplitude at grid
of the sync limiter. This is achieved without reducing the
video bandwidth and the method employed enables the sync
circuits to be isolated from the video feed to the C.R.T.
cathode, again helping preserve bandwidth. The video
detector is an anode bend detector and quite progressive in
the television technique for its period, it re appeared
after the war in a number of Philips T.V. receivers.

The sound system for T.V. is surprising , one would have not
expected to have produced sufficient A.M. sound signal from
the anode of the second common I.F. amplifier to drive the T.V. sound detector directly without
running into problematic intermodulation between sound and
vision signals at 4.5 and 8 MHz. respectively. The secret to achieving a satisfactory result was found to be in
the design of the sound I.F. transformer feeding the
detector, it has a significant step up ratio feeding a much larger signal to the diode detector than appears at the
anode of the I.F valve . A well designed trap in the cathode
of the final video I.F. amplifier ensures total elimination
of the sound signal at the video detector. Only when the
alignment procedure is followed exactly (.this is no easy
task) is adequate volume available, in contrast to this the radio performance is outstanding. The designers saved a
valve by eliminating the need for a separate sound I.F.
amplifier.

Neither of the timebases call for special mention in my opinion,
the circuitry being employed by British firms in one form or
another right up to the introduction of transistors in the late 1960s,critical damping of the line
flyback is used rather than the use of a diode, this was
quite common before and just after the war. Damper diodes
were known and sometimes used however as early as 1937
perhaps even 1936. The method of feeding sync to the line
timebase is a little unusual in that sync of negative
polarity is amplified in the line output valve and fed back
to the grid of the line oscillator in the correct phase,
somewhat unusual and possibly not as good as some methods.
It does however work and shows quite a measure of ingenuity.
Remember all this only ten or so years after the
introduction of sound radio.

The switching system employed in the 904 was nothing less than a
masterpiece of engineering and design to enable normal radio
techniques and the new very high frequency and wide bandwidth circuits to share common
valves. The switching is obvious but very complex for it
time, it performs faultlessly, it would be difficult to improve upon even today employing valves, the valves
themselves could of course be improved upon but that does
not detract from the brilliant circuitry. The 904 being described is almost a direct copy of the original. The
change is minimal and was caused by an error occaisioned
during construction involving the placement of a shield
between switch sections. The performance is not compromised.
At the time of finishing this article a replacement 5FP4 has
been fitted with excellent results. The magnetic focusing
seems better than the electrostatic method employed with the
5AXP4 at the low E.H.T. employed